In a gauge for measuring internal diameters of small bores, small gauging errors arising from the design or manufacture of the gauge itself become intolerably large, whereas in larger diameter bore gauges or other gauging functions the same errors would be inconsequential. For example, an error of 0.0005 inches in a small bore gauge having a range of 0.030 inches, while very small, is completely unacceptable, though possibly acceptable in gauges having a much wider range of movement.
In the design of bore gauges, it is known in the art to provide a gauge of essentially cylindric shape having a tubular work probe provided with a multiplicity of flexible fingers at the end of the probe which are radially displaced to engage the interior of the bore to perform the gauging function. The fingers are provided with conical or tapered surfaces on their distal inner edges with such surfaces of all the fingers being simultaneously engaged by the conical surface of a wedge cam on the end of a plunger running axially of the probe such that axial motion of the plunger causes radial expansion of the distal ends of the fingers until the fingers are restrained by the bore surface to be gauged. The axial movement between the plunger and the probe is measured by a dial indicator or other suitable sensing means to provide a comparative measure of the work diameter. Gauges of this type have inherent errors in their design and it is the primary object of the present disclosure to eliminate such design errors and provide a gauge of improved performance, quality, convenience and fabricational design.